1. Field of the Art
This invention relates to a method for preparing a chitin oligomer or a chitosan oligomer and for preparing a chitosan oligosaccharide which is a mixture of chitin oligomers or a chitosan oligosaccharide which is a mixture of chitosan oligomers at high yield from chitin or chitosan.
2. Related Art
In recent years, food life has become rich, and crabs, prawns are imported from abroad, and from the chitin material of their shells, chitin and chitosan have been produced in large amounts. Chitin and chitosan are themselves now under development as agricultural chemicals, artificial skin, life-related substances, etc., and chitin oligosaccharide or chitosan oligosaccharide which is of further added value is now attracting attention.
It has been known in the art that such chitin oligosaccharide or chitosan oligosaccharide can be generally prepared by degradation with chitin with chitinase or chitosan with chitosanase.
It has been also known that chitin and chitosan can be partially hydrolyzed with hydrochloric acid to prepare from N-acetylglucosamine or glucosamine, N-acetylchitopentaose or chitopentaose.
On the other hand, a method is also known of utilizing the Van Slyke Method quantitating amino-form nitrogen, in which chitosan is depolymerized by addition of nitrous acid to chitosan at a temperature of 20.degree. to 25.degree. C. (U.S. Pat. No. 3,922,260).
However, the above degradation method with an enzyme can control the molecular weight with difficulty, whereby an oligosaccharide mixture having various molecular weights is obtained, and yet the concentration is as dilute as 0.001% and therefore must be concentrated. Accordingly, for obtaining an oligosaccharide with an adequate molecular weight, fractionation must be carried out which is extremely cumbersome laborious, and also since oligosaccharide with a terminal end of the structure as follows: ##STR3## is formed, when heated in the concentration operation or sterilization operation, a Maillard reaction may occur which causes coloration. Also, even if this may be further reduced with a reducing agent, there is the problem that it may be subjected to ring opening to be converted to a sugar alcohol. Besides, this method employs a dilute reaction concentration, and large scale equipment is required for bulk production.
Also, since the above-mentioned method of partially hydrolyzing with hydrochloric acid gives the same product as the above enzyme, the problems of coloration, ring opening as mentioned above cannot be solved.
On the other hand, the latter method of depolymerizing chitosan with nitrous acid, partly because chitosan with an extremely high molecular weight is used, requiring nitrous acid employed to be added in of as much as 3 to 5 moles per 1 mole of the amino groups of the chitosan, whereby it may be degraded to glucosamine which is a monosaccharide. Alternatively, since the decomposition reaction is carried out at a relatively high temperature of 20.degree. to 25.degree. C., side reactions such as transfer reaction may occur before degradation to monosaccharides, and these reaction products may be recombined to form products other than chitosan oligosaccharides.
Accordingly, in the product obtained by such a method, recombined glucoside compounds may be present, other than the above-mentioned glucosamine, which may exhibit toxicity; also unreacted nitrous acid is mixed in the product, whereby there are problems for use as food additives or pharmaceuticals.